Acquisitions - Acquisition Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	Oct. 12, 2018	Jan. 06, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets
Goodwill			$ 728,809	$ 697,608	$ 340,879	$ 319,007
Northern States Financial Corporation
Assets
Cash and due from banks and interest-bearing deposits in other banks	$ 160,145
Equity securities	3,915
Securities available-for-sale	47,149
FHLB and FRB stock	554
Loans	284,924
OREO	2,549
Investment in BOLI	11,104
Goodwill	29,343
Other intangible assets	12,230
Premises, furniture, and equipment	7,039
Accrued interest receivable and other assets	19,717
Total assets	578,669
Liabilities
Noninterest-bearing deposits	346,714
Interest-bearing deposits	116,446
Total deposits	463,160
Borrowed funds	18,218
Senior and subordinated debt	8,038
Accrued interest payable and other liabilities	5,950
Total liabilities	495,366
Consideration Paid
Common stock (2018 - 3,310,912, shares issued at $25.16 per share, 2017 - 21,057,085 share issued at $25.34 per share), net of issuance costs	83,303
Cash paid	0
Total consideration paid	$ 83,303
Common stock, shares issued (in shares)	3,310,912
Shares issued (in dollars per share)	$ 25.16
Standard Bancshares, Inc.
Assets
Cash and due from banks and interest-bearing deposits in other banks		$ 102,149
Equity securities		0
Securities available-for-sale		214,107
FHLB and FRB stock		3,247
Loans		1,762,303
OREO		8,424
Investment in BOLI		55,629
Goodwill		345,334
Other intangible assets		31,072
Premises, furniture, and equipment		56,517
Accrued interest receivable and other assets		60,278
Total assets		2,639,060
Liabilities
Noninterest-bearing deposits		675,354
Interest-bearing deposits		1,348,520
Total deposits		2,023,874
Borrowed funds		0
Senior and subordinated debt		0
Accrued interest payable and other liabilities		34,471
Total liabilities		2,058,345
Consideration Paid
Common stock (2018 - 3,310,912, shares issued at $25.16 per share, 2017 - 21,057,085 share issued at $25.34 per share), net of issuance costs		533,590
Cash paid		47,125
Total consideration paid		$ 580,715
Common stock, shares issued (in shares)		21,057,085
Shares issued (in dollars per share)		$ 25.34